A condenser microphone is mounted with an impedance converter because the impedance of an acoustoelectric converter formed by a diaphragm and a backplate in a unit is high. In most cases, a field effect transistor (FET) is used as the impedance converter, but a vacuum tube is used on rare occasions.
Also, to fulfill the performance and function as a microphone, a sound output circuit, a power supply circuit, a low cut circuit, and the like are provided in the microphone. For a microphone for conference, a microphone for choir, a tie clip microphone attached to the chest part, and the like, the microphone must be inconspicuous.
For this reason, the condenser microphone is separated, in terms of configuration, into a microphone unit, which includes the acoustoelectric converter formed by the diaphragm and the backplate and the impedance converter, and a power module section (output module section) including the sound output circuit, the power supply circuit, the low cut circuit, and the like, and the microphone unit and the power module section are connected to each other by a dedicated microphone cord.
In this case, as the dedicated microphone cord, a two-core shield covered wire is used. This two-core shield covered wire includes a power wire for supplying power from the power module section to the microphone unit, a signal wire for giving sound signals, which are generated from the impedance converter in the condenser microphone unit, to the power module section, and a shielding wire that electrostatically shields the power wire and the signal wire and connects them to the ground.
This dedicated microphone cord is vulnerable to noise (disturbance) coming from the outside because the sound signals are transmitted unbalancedly. Therefore, if strong electromagnetic waves are applied to the dedicated microphone cord, the electromagnetic waves intrude into the microphone unit, being detected by the impedance converter, and therefore noise is sometimes generated.
In recent years, cellular phones have come into wide use rapidly. In the case where a cellular phone is used near a microphone, the microphone receives considerably strong electromagnetic waves (for example, within the range of about several centimeters to several tens centimeters, a field intensity reaching tens of thousands times of field intensity produced in the city by commercial electric waves). Therefore, in the field of microphone, measures against cellular phones are urgently needed.
Conventionally, when the microphone cord is drawn into the microphone unit and is connected to a circuit board on which the impedance converter is mounted, a knot for preventing coming-off is formed in the microphone unit. However, the length of wiring in the microphone unit increases accordingly, so that the electromagnetic waves easily intrude into the microphone unit through the microphone cord.
The applicant of the invention has proposed a condenser microphone unit that takes measures to solve the above problems as Patent Document 1 (Japanese Patent Application Publication No. 2006-74107). The configuration of the proposed condenser microphone unit is explained by reference to FIG. 4.
This condenser microphone unit includes a microphone capsule 10 and a capsule support 20 as a basic configuration, and is connected to a power module section, not shown, via a dedicated microphone cord 30.
The microphone capsule 10 includes a cylindrical capsule case 11 formed of, for example, a brass material. In the housing (cupsule case) 11, a diaphragm 12 stretchedly provided on a support ring 13 and a backplate 14 supported on an insulating seat 15 are housed in the state of being arranged opposedly via an electrical insulating spacer (not shown).
The back surface side of the housing 11 is closed by a back lid 16, and a contact pin 17 that is connected to the backplate 14 via a wiring, not shown, protrudes from the back lid 16. Also, on the rear end side of the capsule case 11, an internally threaded cylinder 18 for connecting with the capsule support 20 is fixed so as to be electrically conducting with the capsule case 11.
The capsule support 20 includes a cylindrical housing 201 formed of, for example, a brass material, and the microphone capsule 10 is detachably connected to the capsule support 20 via a ring coupler 27 formed with external threads at the outer periphery thereof.
On one end side (the upper end side in FIG. 4) of the housing 201, internal threads 202 are formed. The lower half of the external threads of the ring coupler 27 is threadedly engaged with the internal threads 202, and the internally threaded cylinder 18 is threadedly engaged with the upper half of the external threads of the ring coupler 27 in this state, by which the microphone capsule 10 and the capsule support 20 are connected to each other.
Also, the housing 201 is formed with a concave step part 203 at a lower position of the internal threads 202, and as a receiver of the concave step part 203, a circuit board 21 is arranged on one end side of the housing 201 so as to close the interior of the housing 201.
In the example shown in FIG. 4, an FET 22 is mounted on the lower surface side of the circuit board 21 as an impedance converter. The circuit board 21 is a double-sided circuit board. FIG. 5A shows the wiring pattern on the upper surface of the circuit board 21, and FIG. 5B shows the wiring pattern on the lower surface on which the FET 22 is mounted.
The wiring pattern on the lower surface on which the FET 22 is mounted includes lead wirings for electrodes of gate, drain, and source of the FET 22, and the lead wiring for gate of these lead wirings is connected to a gate electrode terminal 211, which is formed in the center on the upper surface side of the circuit board 21, via a wiring in a through hole.
On the upper surface side and the lower surface side in the outer peripheral edge part of the circuit board 21, ground patterns 212a and 212b connected to the drain or source of the FET 22 are formed, respectively, so as to be conducting with each other via a wiring in a through hole.
On the upper surface of the circuit board 21 shown in FIG. 5A, a contact terminal 23 consisting of a plate spring that is in contact with the contact pin 17 is mounted in a state of being held on a spacer 24 of a rubber elastic body. The lower end of the contact terminal 23 is in contact with the gate electrode terminal 211 connecting with the lead wiring for gate of the FET 22.
Thereby, when the microphone capsule 10 is connected to the capsule support 20 via the ring coupler 27, the backplate 14 is connected to the gate of the FET 22 via the contact pin 17, the contact terminal 23, and the gate electrode terminal 211.
Also, the outer peripheral edge part of the circuit board 21 is held between the lower end part of the ring coupler 27 and the concave step part 203 of the housing 201. Thereby, the ground pattern 212a formed on the upper surface side in the outer peripheral edge part of the circuit board 21 is brought into tight contact with the ring coupler 27, and the ground pattern 212b formed on the lower surface side in the outer peripheral edge part of the circuit board 21 is brought into tight contact with the housing 201.
On the other end side (the lower end side in FIG. 4) of the capsule support 20, a cord introduction hole 25 having a cord bush 26 is provided. Through this cord introduction hole 25, the dedicated microphone cord 30 extending from the side of the power module section, not shown, is drawn into the capsule support 20.
As the microphone cord 30, a two-core shield covered wire is used which includes a power wire 31 for supplying power to the microphone capsule 10, a signal wire 32 for sending sound signals generated from the FET 22 to the power module section, not shown, and a net-shaped shield covering wire 33 that electrostatically shields the power wire and the signal wire and connects them to the ground.
According to the invention described in Patent Document 1, the microphone cord 30 is provided with a shield covering wire exposure part 33a, in which the shield covering wire 33 is stripped out, in a portion in which the microphone cord 30 is drawn into the capsule support 20, and a fastening fixture 40 is fixed in the shield covering wire exposure part 33a. 
The shield covering wire exposure part 33a can be formed, for example, by removing the skin on the tip end side of the microphone cord 30 connected to the circuit board 21 to strip out the shield covering wire 33 and by folding back the stripped-out shield covering wire 33.
The fastening fixture 40 is formed as an annularly-shaped body of a washer shape (doughnut shape) that is thick and has a diameter larger than that of the cord introduction hole 25. The outside diameter thereof has a size such that the outside diameter is in close contact with the inner surface of the capsule support 20. Also, on the inner periphery side through which the microphone cord 30 is inserted, a staking sleeve 41 that is fixed to the shield covering wire exposure part 33a by plastic deformation is formed integrally.
When the microphone cord 30 is attached to the capsule support 20, after the microphone cord 30 has been inserted through the cord introduction hole and the tip end side thereof has been drawn out to the outside of the capsule support 20, the fastening fixture 40 is fitted on the shield covering wire exposure part 33a of the microphone cord 30, and the sleeve 41 is staked to fix the microphone cord 30.
After the power wire 31 and the signal wire 32 of the microphone cord have been soldered to a predetermined lead wiring on the circuit board 21, the tip end side of the microphone cord 30 is drawn into the capsule support 20 to engage the circuit board 21 with the concave step part 203 of the housing 201, and also the outer periphery side of the fastening fixture 40 is brought into contact with the inner surface of the capsule support 20.
Thereby, the microphone cord 30 is prevented from coming off, and also the shield covering wire 33 is electrically connected surely to the capsule support 20 via the fastening fixture 40, so that a high-frequency current caused by strong electromagnetic waves applied to the microphone core 30 flows to the capsule support 20 side, and does not intrude into the capsule support 20. Therefore, the generation of noise caused by electromagnetic waves is prevented.
As described above, according to the invention described in Patent Document 1, a knot for preventing coming-off of microphone cord need not be formed in the capsule support 20, and also the shield covering wire 33 is electrically connected surely to the capsule support 20. However, since the microphone capsule 10 and the capsule support 20 are connected to each other via a ring coupler 27, a problem as described below may occur.
When the ring coupler 27 is loosened by vibrations or shocks applied from the outside, or when the microphone capsule 10 is screwed firmly onto the ring coupler 27 with an excessive force, the stress between the ring coupler 27 and the capsule support 20 decreases.
As a result, the ring coupler 27 and the capsule support 20 become in bad contact with the ground patterns 212a and 212b of the circuit board 21, and therefore the shield between the microphone capsule 10 and the capsule support 20 becomes incomplete. Therefore, noise is generated by the high-frequency current, and in an extreme case, a trouble such that the sound signal is broken off occurs.
Accordingly, an object of the present invention is to provide a condenser microphone unit configured by connecting a microphone capsule to a capsule support via a screwed ring coupler, in which shield is maintained stably regardless of the degree of tightening of the ring coupler.